1. Field of the Invention
This invention relates to delayed coking, and more particularly to a method of minimizing the coke yield from a delayed coking operation.
2. The Prior Art
Delayed coking has been practiced for many years. The process broadly involves thermal decomposition of heavy liquid hydrocarbons to produce gas, liquid streams of various boiling ranges, and coke.
Coking of resids from heavy, sour (high sulfur) crude oils is carried out primarily as a means of disposing of low value resids by converting part of the resids to more valuable liquid and gas products. The resulting coke is generally treated as a low value by-product.
The use of heavy crude oils having high metals and sulfur content is increasing in many refineries, and delayed coking operations are of increasing importance to refiners. The increasing concern for minimizing air pollution is a further incentive for treating resids in a delayed coker, as the coker produces gas and liquids having sulfur in a form that can be relatively easily removed.
In the basic delayed coking process as practiced today, feedstock is introduced to a fractionator, and the fractionator bottoms including recycle material are heated to coking temperature in a coker furnace. The hot feed then goes to a coke drum maintained at coking conditions of temperature and pressure where the feed decomposes to form coke and volatile components. The volatile components are recovered and returned to the fractionator. When the coke drum is full of solid coke, the feed is switched to another drum, and the full drum is cooled and emptied by conventional methods.
Some coking operations involve passing vacuum resid directly from a crude oil vacuum distillation unit to a coker furnace with no intermediate storage. An advantage of this method is that the coker feed is always at a readily pumpable temperature, and heated storage or dilution is not required. A disadvantage is that if either the vacuum distillation unit or the coker unit is shut down for any reason, then the other unit must be shut down, or other steps must be taken until the shut down unit is back on stream.
Other coking operations utilize heated or insulated storage tanks to maintain resid at a pumpable temperature. This is probably the preferred design, as it avoids the need for dilution of resid to keep it pumpable, and it provides flexibility if either the distillation unit or the coker unit is temporarily shut down.
Still other coking operations utilize unheated storage of resid. A serious drawback to unheated resid storage is that heavy vacuum resids, such as those having an API gravity of less than about 10, must be diluted with "cutter stock" before they have cooled much below about 300.degree. F., and certainly before they are cooled to 180.degree. F. or so, or else they become so viscous as to be essentially unpumpable. Normally in such feedstock cutting operations a diluent or cutter stock is added to the feed before it is cooled below about 300.degree. F. and before it is placed in an unheated storage tank. In this way, the resid and diluent are well mixed before storage, and can still be pumped out of the storage tank. The major deficiency of this method is that it is energy inefficient, as the resid and cutter stock must be reheated from storage temperature. Also, the volume of diluent required is quite large, requiring larger tanks, pumps, lines, etc.
The present invention is not particularly applicable to those coking operations where diluent is added to resid to maintain its pumpability during storage before it is passed to storage. The invention is primarily beneficial for those coking operations where resid is passed directly to the coker unit from the distillation unit, and to those coking operations where resid is stored at elevated temperature.
The invention is not limited to coking operations where petroleum resid is the feedstock, but is applicable to other coker feedstocks such as coal liquifaction products or other low gravity, high viscosity hydrocarbon streams which might be amenable to delayed coking to produce fuel grade coke.
The delayed coking process is discussed in an article by Kasch et al entitled "Delayed Coking," The Oil and Gas Journal, Jan. 2, 1956, pp. 89-90.
A delayed coking process for coal tar pitches illustrating use of heavy gas oil recycle is shown in U.S. Pat. No. 3,563,884 to Bloomer et al.
A discussion of early delayed coking processes appears in an article by Armistead entitled "The Coking of Hydrocarbon Oils," The Oil and Gas Journal, Mar. 16, 1946, pp 103-111.
U.S. Pat. No. 4,213,846 discloses a delayed coking process for making premium coke in which a recycle stream is hydrotreated.
U.S. Pat. No. 4,216,074 describes a dual coking process of coal liquefaction products wherein condensed liquids from the coke vapor stream and heavy gas oil reflux are used as recycle liquid to the coke drums.
U.S. Pat. No. 4,177,133 describes a coking process in which the heavier material from the coke drum vapor line is combined as recycle with fresh coker feed and then passed to a coke drum.
Many additional references, of which U.S. Pat. Nos. 2,380,713; 3,116,231 and 3,472,761 are exemplary, disclose variations and modifications of the basic delayed coking process.